One of the key events in the cell division cycle is the initiation of chromosomal DNA replication and it is likely that the control of this process in human tumor cells are abnormal. Unfortunately, little is known about the mechanism of initiation of DNA replication in any metazoan species. This project will expand upon an important discovery of a candidate DNA initiator protein that may play a key role in the initiation of chromosomal DNA replication in human cells. The specific goals of the project will be to identify all of the subunits of the human Origin Recognition Complex (hORC) and to study their functions in vitro and in vivo. The sites in the human genome to which hORC binds will be characterized and mapped to regions known to contain origins of DNA replication. Finally proteins that interact with the hORC will be investigated and their interaction with the cell cycle regulatory machinery determined. These studies should advance our understanding of the mechanism of initiation of replication of the human genome and determine how this process is controlled during the cell division cycle. These studies are directly relevant to the overall goals of this program project.